1. Field of the Invention
The present invention generally relates to an optical element, an optical scanner and an image forming apparatus, and more particularly to an optical element that can be preferably used as a writing system of a recording apparatus such as a digital copier, a laser printer and a laser facsimile, an optical scanner using the optical element, and an image forming apparatus using the optical scanner.
2. Description of the Related Art
In recent times, an optical scanner lens being an optical element used in an optical scanner has been fabricated through plastic molding. In order to form a lens through plastic molding, a hot melted plastic material is molded in accordance with a metal pattern and then is cooled in the metal pattern. In the cooling process, a peripheral portion of the metal pattern tends to be cooled earlier than a center portion thereof. As a result, there arises a nonuniform density distribution in the interior of the molded plastics in that a fast-cooled portion has a relatively higher density than a slow-cooled portion. Also, there arises denaturation in the interior of the molded plastics. For this reason, the formed lens cannot have a uniform refractive index in the inner portion thereof, and there arises a refractive index distribution therein.
A description will now be given, with reference to FIG. 1 and FIG. 2, of characteristics of a conventional plastic lens.
FIGS. 1A and 1B show an example of a refractive index distribution in a conventional plastic lens. When an optical scanner lens 6 is virtually cut in a cross-section including the optical axis in parallel to the main scanning direction, FIG. 1A-(a) shows a contour of a refractive index distribution with respect to the cross-section. FIG. 1A-(b) shows the refractive index distribution along the center of the lens thickness as shown in a dashed line in FIG. 1A-(a). In contrast, when the optical scanner lens 6 is virtually cut in a cross-section including the optical axis in parallel to the sub-scanning direction, FIG. 1B-(c) shows a contour of the refractive index distribution with respect to the cross-section. FIG. 1B-(d) shows the refractive index distribution on a surface that includes the optical axis and is parallel to the main scanning direction. FIG. 1B-(e) shows the refractive index distribution along the center of the lens thickness in FIG. 1B-(c). As seen in FIGS. 1A and 1B, the interior of the lens generally has a greater refractive index in the peripheral portion thereof than in the center portion thereof, because the peripheral portion is cooled earlier than the center portion and the peripheral portion has a relatively higher density than the center portion as mentioned above.
In general, when an optical scanner lens contains a refractive index distribution therein, optical characteristics of the optical scanner lens tend to be slightly different from those expected of an optical scanner lens designed to have a uniform refractive index. On average, the optical scanner lens tends to have a higher refractive index in the peripheral portion thereof than in the center portion thereof. In this case, an optical spot, which should be focused on a scanned surface, is actually focused at a position farther away from an expected position with respect to an optical deflector.
Also, when the optical spot scans an effective scanned region of the scanned surface, the diameter of the optical spot varies in accordance with image heights based on a field curvature of the optical scanner lens. However, when the above-mentioned refractive index distribution arises in the interior of the lens, the diameter also varies in accordance with the refractive index distribution.
FIG. 2 shows relations between the diameter of an optical spot and beam defocus in two cases where a lens has a refractive index distribution and no refractive index distribution. Here, a beam defocus means a difference between a focused position (image forming position) of the optical spot and a position of a scanned surface. In FIG. 2, the vertical axis represents the diameter of the optical spot, and the horizontal axis represents an amount of the beam defocus.
In a lens whose refractive index is uniform at any position therein such as a glass lens, that is, a lens that has no refractive index distribution, the optical spot diameter and the defocus amount have the relation as shown by a dotted curve in FIG. 2. As seen in FIG. 2, the optical spot diameter is minimized at a position on the photoreceptor surface, that is, the scanned surface. The beam defocus is 0 at that position. On the other hand, when the lens contains a refractive index distribution, the optical spot diameter and the defocus amount have the relation as shown by a solid curve in FIG. 2. As seen in FIG. 2, the optical spot diameter on the scanned surface is apparently larger than the expected size at the above-mentioned intersection of the dotted curve and the vertical axis due to misalignment of the focused position.
Also, a focused position is not always misaligned by a constant amount for each image height due to refractive index distributions. In the case of a constant misalignment for each image height, if focused positions are adjusted to be located on the dotted curve in FIG. 2, for instance, by moving some components of an optical system in the optical axis direction, it is possible to obtain a proper optical spot for all image heights.
However, when an optical scanner lens in use has a refractive index distribution, the amount of the focused position misalignment is not constant for each image height. Thus, even if a focused position corresponding to a certain image height is adjusted and a proper optical spot is obtained at that position, it is impossible to assure that the adjustment succeeds for other image heights. In particular, when the diameter of an optical spot is narrowed down so that an image forming apparatus can create a higher-quality image, this problem becomes more significant.
If an optical scanner lens is designed without consideration of a refractive index distribution thereof, it is likely that the optical scanner lens cannot create a high-quality image because the optical spot diameter has wide variance with respect to the image heights. Also, when an optical scanner lens with a refractive index distribution is used in a multi-beam optical scanner, the multi-beam optical scanner has a problem in that the multi-beam optical scanner has various pitches between optical spots on a scanned surface for image heights. This problem is caused by the fact that each image height has a different horizontal magnification of image formation between an optical deflector and the scanned surface with respect to the sub-scanning direction. For this problem, when the image heights have a large deviation of horizontal magnifications, there appears an irregularity in the recorded image. In particular, when the pitches of the optical spots are made narrow so that an image forming apparatus can form a high-quality image, the irregularity becomes more significant.
Japanese Laid-Open Patent Applications No. 09-049976, No. 10-288749, No. 11-002768, No. 11-038314, and others disclose some techniques related to an optical scanner whose optical scanner lens is designed with consideration of a refractive index distribution. Also, Japanese Laid-Open Patent Application No. 11-044641 and others disclose means for measuring a refractive index distribution in a lens.
Japanese Laid-Open Patent Application No. 09-049976 discloses an optical scanner lens. The optical scanner lens is formed such that a focal distance computed based on a curvature of the optical scanner lens surface, a refractive index of a material thereof, and a thickness thereof with respect to the optical axis direction is less than a measured focal distance. This optical scanner lens makes it possible to properly correct misalignment of a focused position due to a refractive index distribution in the lens that results from plastic molding of the lens. As mentioned above, when lenses are manufactured in plastic molding, the lenses tend to contain refractive index distributions. However, when the lenses are formed of an identical material under an identical condition on the plastic molding, the lenses have substantially less different refractive index distributions from each other and, therefore, it is possible to obtain information regarding the refractive index distributions through experiments in advance. For this reason, if a shape of the metal pattern in use is corrected based on the information, it is possible to effectively correct the focused position misalignment due to the refractive index distributions.
Regarding the shape correction of the metal pattern, when an amount of the correction is small, it is possible to easily determine with accuracy the shape correction and to easily reshape the metal pattern to be corrected. In this conventional optical scanner lens, however, the curvature and other factors thereof are designed to make the expected focal distance less than the measured focal distance for all image heights so as to eliminate the focused position alignment due to the refractive index distribution on the scanned surface. As a result, it is impossible to avoid a considerably large correction of the metal pattern. Thus, this conventional technique for forming the optical scanner lens has a difficulty with accurate correction. Furthermore, even if the focused position misalignment is properly corrected, a ratio of a deviation F of the focused position misalignment of an optical spot with respect to image heights to an effective write width W scanned by the optical spot on the scanned surface, that is, a ratio F/W, is improved to only at most about 0.007. In order to achieve higher image quality, it is necessary to make the ratio F/W much smaller than 0.007.
As mentioned above, even if an optical scanner lens contains a refractive index distribution, it is possible to obtain proper optical spots for all image heights, for instance, by shifting some components of the optical system in the optical axis direction, as long as focused positions are misaligned by a constant distance for all image heights. In shape correction of a metal pattern, the correction should be intended not to make focused position misalignment uniform for all image heights but to correct a deviation with respect to the image heights as slightly as possible even if the focused position misalignment persists on the scanned surface in this correction. If the metal pattern is reshaped in such a way, it is possible to correct the metal pattern as slightly as possible with high accuracy.
Japanese Laid-Open Patent Application No. 10-288749 discloses an optical scanner lens that has a sufficient depth margin. In such a configuration, even if the optical scanner lens contains a refractive index distribution, it is possible to obtain proper optical spots.
However, when the diameter of an optical spot by the optical scanner lens is made smaller, it becomes difficult to maintain sufficient depth margin. Furthermore, the optical scanner lens has additional problems in that the lens must be shaped and installed under quite severe error constraints thereof. Adversely, these problems increase the fabrication cost. In addition, the optical scanner lens is not preferable in terms of image quality.
Japanese Laid-Open Patent Application No. 11-002768 discloses an optical scanner that can effectively correct misalignment of a focused position of a first optical system.
In this conventional optical scanner, although the first optical system can correct the focused position misalignment for all image heights by an equal correction amount in an identical direction, it is impossible to individually correct the misalignment of each of the focused positions. Thus, only when the refractive index has a significantly narrow distribution and the focused positions are misaligned by a constant for all the image heights, does the optical scanner operate effectively. However, the focused positions are distinctly misaligned for the individual image heights due to the refractive index distribution. In particular, this tendency is remarkable for lenses whose thickness has a large deviation. In this case, it is impossible to obtain proper optical spots.
Japanese Laid-Open Patent Application No. 11-038314 discloses an optical scanner that can effectively correct focused position misalignment due to a refractive index distribution by shifting the focused positions in the negative direction with respect to the image height center portion and in the positive direction with respect to the image height peripheral portion.
When the refractive index has a significantly narrow distribution and the focused positions are misaligned by a constant for all image heights, the optical scanner operates effectively. However, even if practical plastic lenses are formed in the fashion according to this disclosure, it is difficult to obtain proper optical spots.
Here, it should be noted that a deviation of optical spot pitches with respect to image heights is not mentioned in the above-mentioned disclosures.